Remove the damper from your chimney by taking out the pins holding it in place.
Measure the opening of your chimney from inside your fireplace with the tape measure. Add 1 inch to each side of these dimensions and cut a piece of sheet metal to this size with the aviator snips. Snip a notch in each corner of the sheet metal and fold up the edges constructing a flange on each side, 1 inch in length.
Use the tape measure to determine the diameter of your chimney pipe with the tape measure. Cut out a hole in the middle of the sheet metal with the aviator snips. Insert a screwdriver in the middle of the sheet metal to start a hole and finish it with the aviator snips. The hole should equal the diameter of your chimney pipe.
Slip the sheet metal into the underside opening of the chimney so it fits around the chimney pipe that comes down the chimney from above.
Hammer nails through flanges on the sheet metal into the bricks on the side of the chimney to affix the sheet metal around the chimney pipe opening.
Caulk the edges of the sheet metal where it sits against the masonry with the silicone sealant. Also apply the sealant around the edges of the sheet metal where is sits against the chimney pipe.
Insert the bottom of the chimney pipe into your wood stove according to the instructions on your stove. Apply silicone caulk around the joint of the pipe and the chimney pipe.